1. Field of the Invention
The invention relates to a process and apparatus for developing an electrostatic latent charge image on the surface of a recording material by means of a controlled voltage applied to a developing electrode.
2. Description of the Prior Art
There has for a long time been a need, in the development of latent electrostatic charge images, to prevent an adherence of toner to image-free places on original copies of different background brightness, while at the same time, properly developing the image areas.
In electrostatic charge images, as a result of their production, the absolute level of the voltage distribution frequently does not correspond to the values required for optimum development of the image. Thus, for example, the background area of the latent charge image, which ideally should be at zero voltage, can exhibit a residual voltage of 100 to 200 volts. The desired ideal condition can be approximated by increasing the intensity or time of the exposure. Yet, it is more economical and advantageous to effect a compensation for the residual voltage by simply raising the developing voltage by an appropriate amount. This method has the advantage that different background brightness, such as gray tones or color tones of the background of the original, can also be balanced by the compensation voltage.
This principle of controlling the developing voltage can be applied not only in electrophotography with conventional office duplicators, with laser exposure, but also in radiography and in the electrophotographic copying of drawings. In these latter two processes it is not the aim to achieve absolute freedom from background but a certain covering of toner, which permits fine nuances and small differences in density to be better perceived. Electrophotographic development processes in which the greatest achievable brightness, that is to say the maximum freedom from toner, is associated with the zone of lowest potential, are already known and are described fully in the patent literature, for example in U.S. Pat. Nos. 3,674,532, 3,782,818 and 3,892,481; U.S. patent application Ser. No. 57,654, filed July 23, 1970 and in German Offenlegungsschrift No. 2,614,318. In these processes and devices, the residual voltage of the photoconductor is in general determined by a measuring instrument and the measured signal, after suitable electrical processing, is fed to the development unit and/or the developing electrode. Processes of measuring the field, charge, voltage and current can be used in such systems.
The accurate separation, by control engineering, of the control action from the preceding measurement necessitates an expensive design, if accurate results are to be achieved.
In the development process described by U.S. Pat. No. 3,674,532 and Application Ser. No. 57,654, the voltage applied to the developing electrode is controlled in such a way that an electric charge at a voltage depending on the voltage of the charge to be developed is stored and the voltage of the developing electrode is maintained at the value preset by the stored voltage. For this purpose, a component voltage corresponding to a definite percentage of the voltage produced by the magnetic brush from its contact with the charge image, is taken off from a voltage means between the developing electrode, which is, for example, a magnetic brush, and the direct current reference line. This voltage is applied to a capacitor via a relay contact which is normally closed. When this relay contact opens, the energy feed from the voltage means to an amplifier is interrupted so that the latter is thus connected only to the capacitor. A further contact of the relay closes the amplifier circuit and permits the constant voltage stored in the capacitor to be applied to the magnetic brush after suitable amplification. Additionally, a resistor is provided, parallel to the voltage means, between the magnetic brush and the reference line, in order to prevent an undesired increase in voltage on the magnetic brush, which increase would result from the contact of the magnetic brush with a charge area of high voltage on the surface of a photoconductor. As disclosed by this U.S.-Patent the initial zone of the latent charge image has generally already had a chance to leave the region of influence of the developing device after the end of the measurement and the subsequent switchingover to control of the voltage applied to the developing electrode, so that the control voltage obtained via the measurement acts too late to become equally effective for the entire latent charge image. The result is then a copy having a starting edge which is to be ascribed to the fact that the voltage applied to the developing electrode is still uncontrolled at the beginning.
U.S. Pat. No. 3,782,818, discloses a device for preventing development of background areas resulting from an electrostatic residual voltage in a latent electrostatic charge image which has an electrode associated with a development unit and means for applying a preset voltage to the electrode. A detector scanning the surface of the photoconductor drum measures the voltage of the background. The output voltage of the detector is amplified and serves to set the voltage supplied by a source of voltage to the electrode.
The control is effected in a similar way by a device for automatically controlling the voltage of a developer electrode, as disclosed in U.S. Pat. No. 3,892,481 in which a spatial separation between the measuring zone and the developer zone is provided. For this purpose, a first electrode for measuring the voltage of the photoconductor is mounted in a wet developer trough, while the electrodes following in the running direction take over the control of the developing voltage and hence of the course of development.
A developing process and apparatus with an automatic applied voltage which is controlled with the aid of a Zener diode which is parallel to a constant current source in the circuit, is known from German Offenlegungsschrift No. 2,614,318. The Zener diode and the constant current source are connected to the developing electrode. When the average voltage of the latent charge image rises above a preset value, the weak current which is passed from the constant current source to the developing electrode is bridged or shunted by the Zener diode so that the developing electrode is prevented from being charged with an excessively high voltage.